The United States and Russia have reached a deal on their most extensive nuclear arms-control agreement in nearly two decades, the Kremlin announced Wednesday. The pact appeared to represent President Obama's first victory in his ambitious agenda to move toward a nuclear-free world.

The new Strategic Arms Reduction Treaty (START) would replace a 1991 pact that expired in December. Experts called the new agreement the most significant arms-control accord since the 1993 signing of START II, which the Russians never ratified.

Officials in both countries would not discuss details of the new accord, but the general outlines have emerged during the year-long negotiations. Each side will reduce its most dangerous nuclear weapons -- those deployed for long-range missions-- from a ceiling of 2,200 to between 1,500 and 1,675. And the two militaries will make relatively small cuts in the number of jets and land- or submarine-based missiles that carry nuclear warheads and bombs.

A Kremlin spokesman told reporters that the two countries' presidents would talk soon to decide when to sign the pact. "All documents related to the new treaty have been agreed upon," he said, speaking on the condition of anonymity because of diplomatic sensitivity.

The declaration appeared to surprise the White House, with spokesman Robert Gibbs saying that the two sides were "close" to a treaty but that it would not be announced until Obama could speak with President Dmitry Medvedev, probably in the next few days.

Still, U.S. officials confirmed that all major obstacles to the pact have been cleared. Secretary of State Hillary Rodham Clinton said in Moscow last week that only technical details remained. Czech authorities said Wednesday that the U.S. government had asked them to hold a signing ceremony. Sen. Richard G. Lugar (R-Ind.), a key supporter of arms control, told Reuters that the administration hoped to have the ceremony on April 8 in Prague, where Obama first laid out the arms-control agenda that helped win him the Nobel Peace Prize. The president briefed Senate Foreign Relations Chairman John F. Kerry (D-Mass.) and Lugar, the panel's ranking Republican, on the pact Wednesday.

While Americans may think of countries such as Iran or North Korea as more dire threats, Russia is still the only nation with the nuclear heft to obliterate the United States. The former Cold War enemies own nearly 95 percent of the world's nuclear weapons, even after extensive reductions in recent years.

"I think there was a real question about whether or not we could do arms control anymore -- whether we had the institutional capacity, and whether the interests of the countries aligned in such a way they could do it," said Jeffrey G. Lewis, a nuclear-weapons specialist at the New America Foundation.

Perhaps more important for Obama, the accord comes shortly before a crucial meeting of signers of the Non-Proliferation Treaty, the global pact that contained the spread of nuclear weapons for decades. The administration is hoping to persuade treaty members to impose stiffer punishments on nations that are accused of violating the pact.

Steven Pifer, a nuclear-weapons expert at the Brookings Institution, said the administration will make the case that, with the new START treaty, "the United States and Russia are doing their part to reduce nuclear weapons. That will give the administration a strengthened hand" in the conference.

Despite some arms-control activists' jubilation, though, the New START, as it is known, probably will face a fight in the Senate, where it will need 67 votes to be ratified. Minority Leader Mitch McConnell (Ky.) and other Republican senators have warned that they will not approve a treaty that would curb a planned U.S. missile defense system for Europe.

The Russian government strongly opposes the missile shield, fearing it will upset its strategic balance with Washington. The U.S. government says the system is aimed at threats from Iran, not Russia.

In recent weeks, Russian negotiators pressed for tougher language in the treaty on the U.S. missile shield. The American side "pushed back very hard on this, in part because the administration has read the tea leaves in the Senate," Pifer said.

Another GOP concern focuses on measures to allow each side to verify the other's deployed weapons. The Russians had balked at continuing some provisions from the first START pact, saying they were too intrusive.

It could be months before the treaty goes to the Senate, because U.S. and Russian negotiators still must draw up technical annexes.

"The White House will be lucky if it can get something signed in time for the Senate to ratify it before the 2010 elections, and the treaty could easily get hung up in partisan debate," Henry Sokolski, a nuclear-weapons expert and executive director of the Nonproliferation Policy Education Center, wrote recently in the National Review Online.

He noted that it took 430 days to ratify the 1991 START agreement. The 2002 Moscow Treaty, a three-page document that committed each side to cut deployed weapons but included no verification measures, took more than nine months.

Obama was deeply involved in the START talks, meeting several times with Medvedev and hashing out differences by telephone, senior officials said.

About three weeks ago, the officials said, the Russians expressed frustration and suggested breaking off the talks for a month. But the United States pressed to continue the negotiations. The administration sent Ellen O. Tauscher, the undersecretary of state for arms control, to Geneva to help seal the deal, they said.